Okay, so this is entirely new to me. Sparrow is was an email client for Mac OS X and iOS (and Windows), which brought a decent Gmail experience to these platforms - as opposed to Apple's own not-so-good Gmail support and Google's Gmail iOS application which, well, is just a webpage. Google has now acquired Sparrow, and basically all hell has broken loose, to the point of Rian van der Merwe writing that 'we' lost "faith in a philosophy that we thought was a sustainable way to ensure a healthy future for independent software development, where most innovation happens".

If you have the budget, you can just hire a team of developers to recreate it with the features you need. Takes a little more effort, but just because there is one closed source app, it does not prevent you from creating another closed or open source app.